An electroexplosive device (EED) is an initiator or a system in which an electrical impulse initiates detonation or deflagration of an explosive. As used herein, the term EED is intended to include electric blasting caps (EBC). An EED generally includes a dc power source electrically connected to a firing means via input firing leads. The firing means is of the type which fires an ignition mixture when sufficient current is applied to the firing means. EEDs are used in both the military market and in the civilian market for blasting applications, for ammunition applications as well as for air bags or the like. Because EEDs can rapidly generate large volumes of gas, they also can be used in conjunction with nearly any item which must be rapidly inflated.
The new and varied uses being found for EEDs has generated a concomitant demand for greater precision and accuracy in the operation of such EEDs. As discussed in the 175th Anniversary Edition of the Blasters' Handbook published by the Sales Development Section Explosives Products Division of E. I. duPont Nemours & Co in 1977, the disclosure of which is incorporated herein by reference, one primary reason for failures in blasting operations is the inaccuracy associated with the explosives themselves. Precise detonation of explosives can decrease the disadvantages associated with rubble, air blast, vibration, noise, fly rock, and the like. Those skilled in the art realize that the more precise and accurate the control of the explosive detonation, the more precise and accurate the overall blasting operation can be.
For example, if the delay associated with the firing of an EED can be known with extreme precision, the spacing and pattern associated with a particular blasting operation can be set with equal precision. Other advantages of precise control over the detonation of the explosives used in a blasting operation can be understood from the aforementioned Blasters' Handbook.
For this reason, the art has included various initiating devices that are intended to provide precise control over the initiation of an explosive. Some of these control devices are discussed in Chapters 7 and 24 of the Blasters' Handbook. Further control over the initiation process can be obtained using controlled blasting machines such as manufactured by Research Energy of Ohio, Inc., and discussed in REO Technical Data Bulletin for DC450-50J, CD450A-50J, CD600-100J and CD700-69J. Other sequential blasting machines such as the REO BM175-10ST can be used as can programmable machines such as the REO BM175-10PT machine, or the like.
While these control means have been successful in the past, the increased strictness of the precision requirements placed on the explosive devices both by the blasting applications of EEDs and by the non-blasting applications of EEDs, have resulted in most prior art initiation control means being at a disadvantage.
Therefore, the art has included initiation control means that are capable of extremely accurate and precise control functions. One such control means is the igniter disclosed in U.S. Pat. No. 4,708,060 (the disclosure of which is incorporated herein by reference, and hereinafter referred to as '060). The control means disclosed in this patent includes a non-electrically conducting substrate supporting an electrical material having a negative coefficient of electrical resistivity at an elevated temperature and which defines a pair of spaced apart pads and a connecting bridge having a resistance of less than about three ohms. A major requirement of for this material is that it develop a temperature coefficient of electrical resistivity which is negative at some temperature, e.g., some temperature above room temperature, e.g., about 100.degree. C. The precise temperature is not critical. Essentially all semiconductors will have this property at sufficiently high doping levels. In general, it is preferred that the semiconductor material be doped essentially at or near its saturation level, e.g., approximately 10.sup.19 atoms/cc, e.g. phosphorus atoms for n-type silicon. Lower doping levels may also be operable under appropriate conditions which can be determined routinely in accordance with the guidelines given in this disclosure. For example, doping levels lower by a factor of 2 from this value will also provide adequate properties for the purposes of this invention. Corresponding resistivity values will be in the order of 10.sup.-3 to 10.sup.-4 e.g., about 8.times.10.sup.-4 .OMEGA.cm for the mentioned saturation doping level. However, other than as explained above, resistivity values per se are not critical. An electrical conductor is connected to each metallized layer and explosive material covers the device. When an electrical current passes through the device, the bridge bursts, igniting the explosive material associated with the device. If this patented initiating control device is used, especially if in conjunction with accurate control devices such as the aforementioned REO, Inc device, extremely accurate and precise control of the explosive initiation process can be obtained.
While the patented igniter can be extremely accurate, this device has virtually no means for preventing inadvertent ignition of the associated explosive charge due to extraneous electricity incident on the overall EED. Therefore, use of this patented igniter may tend to increase the risk of inadvertent initiation due to entry of extraneous electricity into the circuit. As discussed in the parent patent application and in the Blasters' Handbook, extraneous electricity can be created by stray ground currents, lightning and static electricity from electrical storms, radio frequency sources in the vicinity, induced currents caused by electromagnetic field sources in the vicinity, static electricity generated by wind-transported dust, moving equipment and moving equipment parts, and the like, as well as galvanic currents generated by dissimilar metals touching each other or touching a common conductive material.
The accepted "safe" level of extraneous electricity in EEDs is a function of current required to detonate an EED. If extremely small EEDs are used, as could be the case due to the precise control associated with the abovementioned systems, the safe level of extraneous electricity incident on an EED may be reduced from the 50 milliampres suggested by the Institute of Makers of Explosives (IME).
While the art has included several suggestions for preventing extraneous electricity from inadvertently detonating an EED, many such systems may not be adequate for such small and accurate EEDs as may be possible as a result of the combination of the patented igniter and devices such as the REO, Inc device. For example, prior art means for preventing extraneous electricity from inadvertently activating an EED including safety techniques and procedures, measuring and monitoring devices, and the like, may work well for large EEDs, but may not be adequate for small, precise EEDs.
Tests on the device disclosed in the parent application have shown that device to be extremely effective in preventing passage of current generated by extraneous electricity generated from sources such as RF energy incident on the EED and static electricity to a firing means associated with the EED. For example, tests conducted by the Franklin Institute have found the device disclosed in the parent application to prevent firing electrical current to be generated in the EED firing means even though that EED is subjected to RF energy of as high as 19 watts.
Accordingly, accurate, precise, and perhaps, small EEDs can be protected if they can be combined with the device disclosed in the parent application and the grandparent patent.